Bacillus thuringiensis (Bt) produces an insect toxin designated as .delta.-endotoxin. It is synthesized by the Bt sporulating cell. The toxin, upon being ingested in its crystalline form by susceptible insect larvae, is transformed into biologically active moieties by the insect gut juice proteases. The primary target is insect cells of the gut epithelium, which are rapidly destroyed.
The reported activity spectrum of Bt covers insect species within the order Lepidoptera, many of which are major pests in agriculture and forestry. The activity spectrum also includes the insect order Diptera, which includes mosquitoes and black flies. See Couch, T.L., (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C.C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, et al., Z. ang. Ent.(1983) 96:500-508, describe a Bt isolate named Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
In European Patent Application 0 202 739 there is disclosed a novel Bt isolate active against Coleoptera. It is known as B. thuringiensis var. san diego (B.t.s.d.).
In the order Lepidoptera there are pest species that, in the larval stage, feed on foilage of crop plants. The larvae of many of these species are readily controlled by spraying foliage with commercial B.t. insecticides based on lepidopteran-active strains of B.t., such as the HD-1 strain. Coleopteran-active strains, such as B.t s.d., can be used in a similar manner to control foliar-feeding beetles. The Colorado potato beetle (Leptinotarsa decemlineata), for example, is susceptible to the delta-endotoxin of B.t.s.d. and larvae are killed upon ingesting a sufficient dose of spore/crystal preparation on treated foliage.
There are many agricultural crops which are attacked by pest species from both the order Coleoptera (beetles) and the order Lepidoptera (caterpillars). One of the limitations of currently available B.t. insecticides is that they can control species from one order or the other but not both. Thus a B.t. insecticide that can control both types of pests on a given crop would be very beneficial. Several examples of insect control where such a product could be very useful are given below.
Stored grains or other stored products such as flour, nuts, dried fruit, cereals, and the like, are attacked by a broad range of insect species. Most of these are either beetles or moths belonging to the orders Coleoptera and Lepidoptera, respectively. B.t. has been shown to be active against many of the moths attacking stored products. However, a serious limitation to its widespread use in controlling these Lepidopteran pests has been the selectivity of standard commercial B.t. preparations, i.e., they kill only Lepidopteran species and do not affect the beetles that are often present at the same time. For example, it would be desirable to have a pesticide that would be effective against the Indian meal moth, Plodia interpunctella, and the Mediterranean flour moth, Anagasta kulniella, as well as the flour beetles, Tribolium castaneum and Tribolium confusum, and, thus simultaneously control two of the major stored product pests in milled grain products.
The Mexican bean beetle, Epilachna varivestis, is a major pest of beans in the U.S. and Mexico. The adults and larvae feed on the foliage of bush and pole beans as well as on soybeans. In soybeans there is a complex of lepidopteran pests that includes the velvetbean caterpillar, Anticarsia gemmatalis, and the soybean looper, Pseudoplusia includens. A product that could control the bean beetle as well as the caterpillars occurring on soybeans would represent a major improvement over conventional products.
A number of crops are attacked by flea beetles. These beetles belong to the family Chrysomelidae, the same family as the Colorado potato beetle, Leptinotarsa decemlineata. The adults can cause extensive damage by feeding on the foliage. In many of these crops, lepidopteran defoliators also occur. Specific examples include cabbage and tobacco. In cabbage and related crucifers there are flea beetle species in the genus Phyllotreta that are serious problems. At the same time there are several caterpillars that are the major pests in cabbage. These include the cabbage looper, Trichoplusia ni; the diamondback moth, Plutella xylostella; and the imported cabbage worm, Pieris rapae. A product that could control the lepidopteran pest complex as well as the flea beetle species would have an obvious advantage over a more restricted B.t. product. A similar situation occurs in tobacco, with certain Epitrix spp. of flea beetle and the tobacco hornworm, Manduca sexta.
The above clearly depicts the need for an insecticide which can be used to control both coleopteran and lepidopteran pests.